Software cost estimation

1. Software cost estimation • Predicting the resources required for a software development process

2. Topics covered • Productivity • Estimation techniques • Algorithmic cost modelling • Project duration and staffing

3. Fundamental estimation questions • How much effort is required to complete an activity? • How much calendar time is needed to complete an activity? • What is the total cost of an activity? • Project estimation and scheduling and interleaved management activities

4. Software cost components • Hardware and software costs • Travel and training costs • Effort costs (the dominant factor in most projects) • salaries of engineers involved in the project • Social and insurance costs • Effort costs must take overheads into account • costs of building, heating, lighting • costs of networking and communications • costs of shared facilities (e.g library, staff restaurant, etc.)

5. Software pricing factors

6. Productivity measures • Size related measures based on some output from the software process. This may be lines of delivered source code, object code instructions, etc. • Function-related measures based on an estimate of the functionality of the delivered software. Function-points are the best known of this type of measure

7. Measurement problems • Estimating the size of the measure • Estimating the total number of programmer months which have elapsed • Estimating contractor productivity (e.g. documentation team) and incorporating this estimate in overall estimate

8. Function points • Based on a combination of program characteristics • external inputs and outputs • user interactions • external interfaces • files used by the system • A weight is associated with each of these • The function point count is computed by multiplying each raw count by the weight and summing all values

9. Function points • Function point count modified by complexity of the project • FPs can be used to estimate LOC depending on the average number of LOC per FP for a given language • LOC = AVC * number of function points • AVC is a language-dependent factor varying from 200-300 for assemble language to 2-40 for a 4GL • FPs are very subjective. They depend on the estimator. • Automatic function-point counting is impossible

10. Object points • Object points are an alternative function-related measure to function points when 4Gls or similar languages are used for development • Object points are NOT the same as object classes • The number of object points in a program is a weighted estimate of • The number of separate screens that are displayed • The number of reports that are produced by the system • The number of 3GL modules that must be developed to supplement the 4GL code

11. Object point estimation • Object points are easier to estimate from a specification than function points as they are simply concerned with screens, reports and 3GL modules • They can therefore be estimated at an early point in the development process. At this stage, it is very difficult to estimate the number of lines of code in a system

12. Factors affecting productivity

13. Estimation techniques • There is no simple way to make an accurate estimate of the effort required to develop a software system • Initial estimates are based on inadequate information in a user requirements definition • The software may run on unfamiliar computers or use new technology • The people in the project may be unknown • Project cost estimates may be self-fulfilling • The estimate defines the budget and the product is adjusted to meet the budget

14. Estimation techniques • Algorithmic cost modelling • Expert judgement • Estimation by analogy • Pricing to win

15. Algorithmic code modelling • A formulaic approach based on historical cost information and which is generally based on the size of the software • Most commonly used product attribute for cost estimation is code size

16. Expert judgement • One or more experts in both software development and the application domain use their experience to predict software costs. Process iterates until some consensus is reached. • Advantages: Relatively cheap estimation method. Can be accurate if experts have direct experience of similar systems • Disadvantages: Very inaccurate if there are no experts!

17. Estimation by analogy • The cost of a project is computed by comparing the project to a similar project in the same application domain • Advantages: Accurate if project data available • Disadvantages: Impossible if no comparable project has been tackled. Needs systematically maintained cost database

18. Pricing to win • The project costs whatever the customer has to spend on it • Advantages: You get the contract • Disadvantages: The probability that the customer gets the system he or she wants is small. Costs do not accurately reflect the work required

19. Pricing to win • This approach may seem unethical and un-businesslike • However, when detailed information is lacking it may be the only appropriate strategy • The project cost is agreed on the basis of an outline proposal and the development is constrained by that cost • A detailed specification may be negotiated or an evolutionary approach used for system development

20. Management options

21. Project duration and staffing • As well as effort estimation, managers must estimate the calendar time required to complete a project and when staff will be required • The time required is independent of the number of people working on the project

22. Staffing requirements • Staff required can’t be computed by diving the development time by the required schedule • The number of people working on a project varies depending on the phase of the project • The more people who work on the project, the more total effort is usually required • A very rapid build-up of people often correlates with schedule slippage